Problem: Kwesi is putting on sunscreen. He uses $3\text{ ml}$ to cover $45 \text{ cm}^2$ of his skin. He wants to know how many milliliters of sunscreen $(g)$ he needs to cover $240\text{ cm}^2$ of his skin. He assumes the relationship between milliliters of sunscreen and area is proportional. How many milliliters of sunscreen does Kwesi need to cover $240 \text{ cm}^2$ of his skin?
We can set up a proportion like this: $\dfrac{\text{45 cm}^2}{\text{240 cm}^2} = \dfrac{\text{Sunscreen needed to cover 45 cm}^2}{\text{Sunscreen needed to cover 240 cm}^2}$ Substituting values from the problem, we get this: $\dfrac{45\text{ cm}^2}{240\text{ cm}^2} = \dfrac{3\text{ ml}}{g\text{ ml}}$ Let's solve for $g$ : $\dfrac{45\text{ cm}^2}{240\text{ cm}^2} = \dfrac{3\text{ ml}}{g\text{ ml}}$ $g \cdot 45 = 3 \cdot 240 $ $45g = 720$ $g = \dfrac{720}{45}~~~~~~~~~~~$ Divide both sides by $45$. $g = 16$ Kwesi needs $16 \text{ ml}$ of sunscreen to cover $240 \text{ cm}^2$ of his skin.